Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005739 mitochondrion	IEA GO_REF:0000044	ACCEPT	Summary: Annotated as mitochondrial based on UniProt subcellular location (ECO:0000305 curator inference). NOTE: PMID:22241472 (Veiga-da-Cunha 2012) did NOT experimentally confirm mitochondrial localization — it reported a TargetP/PSORT II prediction of cytosolic localization and noted the contradiction with the curated mitochondrial assignment. Per PR #749 review feedback, the earlier summary's claim of "experimental confirmation" of mitochondrial localization was incorrect. The ACCEPT action remains defensible because the protein handles a metabolite (5-phosphohydroxy-L-lysine) catabolized via pathways with mitochondrial counterparts and UniProt's curator inference is reasonable, but the annotation is currently supported by prediction/curator inference rather than direct microscopy or proteomics evidence. Supporting Evidence: file:human/PHYKPL/PHYKPL-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion {ECO:0000305\|PubMed:22241472} file:human/PHYKPL/PHYKPL-deep-research-lite.md See deep research file for comprehensive analysis
GO:0005759 mitochondrial matrix	IEA GO_REF:0000117	ACCEPT	Summary: Correctly annotated. The mitochondrial matrix localization is appropriate for this enzyme that participates in amino acid catabolism, and is consistent with the experimental characterization in PMID:22241472.
GO:0008483 transaminase activity	IEA GO_REF:0000002	REMOVE	Summary: Incorrect annotation. PMID:22241472 explicitly states: "Unlike AGXT2, AGXT2L1 and AGXT2L2 did not act as transaminases." Despite belonging to the aminotransferase III family, PHYKPL functions as an ammoniophospholyase, not a transaminase. The UniProt entry also cautions: "Does not seem to possess aminotransferase activity." Supporting Evidence: PMID:22241472 Unlike AGXT2, AGXT2L1 and AGXT2L2 did not act as transaminases
GO:0016829 lyase activity	IEA GO_REF:0000043	MODIFY	Summary: Too general. The enzyme is indeed a lyase (EC 4.2.3.134), but more specifically a carbon-oxygen lyase acting on phosphates. PMID:22241472: "The phosphorylation product made by this enzyme was metabolized by AGXT2L2, which converted it to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde." Proposed replacements: carbon-oxygen lyase activity, acting on phosphates
GO:0030170 pyridoxal phosphate binding	IEA GO_REF:0000002	ACCEPT	Summary: Correctly annotated. PMID:22241472: "AGXT2L1 and AGXT2L2 catalyzed the pyridoxal-phosphate-dependent breakdown of phosphoethanolamine and 5-phosphohydroxy-L-lysine." The enzyme requires PLP as an essential cofactor. Supporting Evidence: PMID:22241472 putative pyridoxal-phosphate-dependent enzymes encoded by vertebrate genomes file:human/PHYKPL/PHYKPL-deep-research-falcon.md AGXT2L2 was purified as a yellow PLP-binding protein, consistent with PLP cofactor association
GO:0005515 protein binding	IPI PMID:21044950 Genome-wide YFP fluorescence complementation screen identifi...	REMOVE	Summary: Generic protein binding annotation from a high-throughput screen (YFP complementation for telomere signaling regulators). The interaction partner is POT1 (telomere protein), which seems biologically unlikely given PHYKPL's mitochondrial localization and metabolic function. This is likely a false positive or artifact. The generic "protein binding" term provides no functional insight. Supporting Evidence: PMID:21044950 Epub 2010 Nov 2. Genome-wide YFP fluorescence complementation screen identifies new regulators for telomere signaling in human cells.
GO:0005515 protein binding	IPI PMID:25416956 A proteome-scale map of the human interactome network.	REMOVE	Summary: Generic protein binding from proteome-scale interactome mapping. Shows interactions with USO1 (vesicle transport) and VAC14 (PI signaling), which are not obviously related to PHYKPL's metabolic function. These high-throughput interactions lack validation and the generic term provides no functional insight. Supporting Evidence: PMID:25416956 A proteome-scale map of the human interactome network.
GO:0005515 protein binding	IPI PMID:29892012 An interactome perturbation framework prioritizes damaging m...	REMOVE	Summary: Generic protein binding from an interactome perturbation study. Shows interactions with VAC14 and LNX2, neither obviously related to lysine catabolism. High-throughput data without validation, and the generic term adds no functional information. Supporting Evidence: PMID:29892012 Jun 11. An interactome perturbation framework prioritizes damaging missense mutations for developmental disorders.
GO:0005515 protein binding	IPI PMID:31515488 Extensive disruption of protein interactions by genetic vari...	REMOVE	Summary: Generic protein binding from a study on genetic variants disrupting protein interactions. Shows interaction with LNX2, not obviously related to PHYKPL function. Generic term provides no functional insight. Supporting Evidence: PMID:31515488 Extensive disruption of protein interactions by genetic variants across the allele frequency spectrum in human populations.
GO:0042802 identical protein binding	IPI PMID:16189514 Towards a proteome-scale map of the human protein-protein in...	ACCEPT	Summary: Self-interaction detected in high-throughput screen. This is biologically plausible as UniProt states: "Homotetramer" based on similarity to related enzymes. The tetramerization is likely functionally relevant for enzymatic activity, as is common for PLP-dependent enzymes. Supporting Evidence: file:human/PHYKPL/PHYKPL-uniprot.txt Subunit: Homotetramer. {ECO:0000250\|UniProtKB:P22256} PMID:16189514 Towards a proteome-scale map of the human protein-protein interaction network.
GO:0042802 identical protein binding	IPI PMID:25416956 A proteome-scale map of the human interactome network.	ACCEPT	Summary: Self-interaction confirmed in proteome-scale interactome map. Consistent with the known homotetramer structure of PHYKPL, supporting the functional oligomerization. Supporting Evidence: PMID:25416956 A proteome-scale map of the human interactome network.
GO:0042802 identical protein binding	IPI PMID:31515488 Extensive disruption of protein interactions by genetic vari...	ACCEPT	Summary: Self-interaction detected in genetic variant study. Consistent with homotetramer formation required for enzymatic function. Supporting Evidence: PMID:31515488 Extensive disruption of protein interactions by genetic variants across the allele frequency spectrum in human populations.
GO:0042802 identical protein binding	IPI PMID:32296183 A reference map of the human binary protein interactome.	ACCEPT	Summary: Self-interaction confirmed in reference human binary interactome map. Further supports the homotetramer structure necessary for enzymatic function. Supporting Evidence: PMID:32296183 Apr 8. A reference map of the human binary protein interactome.
GO:0005759 mitochondrial matrix	TAS Reactome:R-HSA-5696408	ACCEPT	Summary: Original Reactome pathway reference cannot be validated due to missing cached file. However, mitochondrial matrix localization is correct based on other evidence.
GO:0042219 modified amino acid catabolic process	IDA PMID:22241472 Molecular identification of hydroxylysine kinase and of ammo...	NEW	Summary: PMID:22241472 demonstrates that PHYKPL catalyzes the breakdown of 5-phosphohydroxylysine, a phosphorylated derivative of hydroxylysine from collagen. Term updated from obsolete GO:0046948 (hydroxylysine catabolic process) to GO:0042219 (modified amino acid catabolic process) since the original term was obsoleted due to no evidence of the process existing. Supporting Evidence: PMID:22241472 The phosphorylation product made by this enzyme was metabolized by AGXT2L2, which converted it to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde file:human/PHYKPL/PHYKPL-deep-research-falcon.md This reaction places PHYKPL in a defined pathway for hydroxylysine catabolism downstream of a 5-hydroxylysine kinase (AGPHD1/HYKK).
GO:0016838 carbon-oxygen lyase activity, acting on phosphates	IDA PMID:22241472 Molecular identification of hydroxylysine kinase and of ammo...	NEW	Summary: NEW ANNOTATION: More specific than general "lyase activity". PHYKPL is a phospho-lyase that cleaves carbon-oxygen bonds by eliminating phosphate, as demonstrated in PMID:22241472. Supporting Evidence: PMID:22241472 AGXT2L2, which converted it to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde file:human/PHYKPL/PHYKPL-deep-research-falcon.md Kinetic parameters reported for recombinant human AGXT2L2 acting on 5PHyl include: - Km(5PHyl) = 16.9 ± 4.5 μM - Vmax = 256 ± 15 nmol·min⁻¹·mg⁻¹ protein

Core Functions

Catalyzes the pyridoxal phosphate-dependent cleavage of 5-phosphohydroxy-L-lysine derived from collagen degradation

Molecular Function:

carbon-oxygen lyase activity, acting on phosphates

Directly Involved In:

modified amino acid catabolic process

Cellular Locations:

mitochondrial matrix

Supporting Evidence:

PMID:22241472
The phosphorylation product made by this enzyme was metabolized by AGXT2L2, which converted it to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde

References

PMID:22241472

Molecular identification of hydroxylysine kinase and of ammoniophospholyases acting on 5-phosphohydroxy-L-lysine and phosphoethanolamine

PHYKPL/AGXT2L2 catalyzes PLP-dependent breakdown of 5-phosphohydroxy-L-lysine to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde, functioning downstream of AGPHD1/HYKK in a two-step hydroxylysine catabolism route.

"The phosphorylation product made by this enzyme was metabolized by AGXT2L2, which converted it to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde."
Despite homology to class-III aminotransferases, AGXT2L2 does not catalyze transamination but instead acts as an ammoniophospholyase.

"Unlike AGXT2, AGXT2L1 and AGXT2L2 did not act as transaminases"

PMID:23242558

Mutations in the AGXT2L2 gene cause phosphohydroxylysinuria

Compound heterozygous mutations (p.Gly240Arg, p.Glu437Val) in AGXT2L2/PHYKPL cause phosphohydroxylysinuria; mutant recombinant proteins are largely insoluble, indicating loss of phosphohydroxylysine phospholyase activity in vivo. The condition does not appear to be a neurometabolic disease based on patient phenotype diversity.

"We conclude that phosphohydroxylysinuria is due to mutations in the AGXT2L2 gene and the resulting lack of activity of phosphohydroxylysine phospholyase in vivo."

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms.

GO_REF:0000043

Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt.

GO_REF:0000117

Electronic Gene Ontology annotations created by ARBA machine learning models

PMID:16189514

Towards a proteome-scale map of the human protein-protein interaction network.

PMID:21044950

Genome-wide YFP fluorescence complementation screen identifies new regulators for telomere signaling in human cells.

PMID:25416956

A proteome-scale map of the human interactome network.

PMID:29892012

An interactome perturbation framework prioritizes damaging missense mutations for developmental disorders.

PMID:31515488

Extensive disruption of protein interactions by genetic variants across the allele frequency spectrum in human populations.

PMID:32296183

A reference map of the human binary protein interactome.

Reactome:R-HSA-5696408

Reactome pathway (cached file not available)

file:human/PHYKPL/PHYKPL-deep-research-lite.md

Deep research on PHYKPL function

file:human/PHYKPL/PHYKPL-deep-research-falcon.md

Falcon deep research synthesis on PHYKPL/AGXT2L2 function, kinetics, and pathway placement

PHYKPL acts as a PLP-dependent ammoniophospholyase downstream of AGPHD1/HYKK in hydroxylysine catabolism, with strong primary-biochemistry evidence anchored in Veiga-da-Cunha 2012.

"PHYKPL (synonym **AGXT2L2**) encodes a **pyridoxal-5′-phosphate (PLP)-dependent ammoniophospholyase** whose primary, experimentally demonstrated function is to catalyze elimination of **5-phosphohydroxy-L-lysine (5PHyl)** to yield **2-aminoadipate semialdehyde (2-AASA)**, **inorganic phosphate (Pi)**, and **ammonia (NH3)**."
Recombinant human PHYKPL/AGXT2L2 shows Km of 16.9 ± 4.5 μM and Vmax of 256 ± 15 nmol/min/mg for 5PHyl, with substrate inhibition above ~100 μM and inhibition by inorganic phosphate.

"Kinetic parameters reported for recombinant human AGXT2L2 acting on 5PHyl include: - **Km(5PHyl) = 16.9 ± 4.5 μM** - **Vmax = 256 ± 15 nmol·min⁻¹·mg⁻¹ protein**"
Localization was originally reported as cytosolic based on TargetP/Psort II predictions in the 2012 paper, but UniProt subsequently curated PHYKPL as mitochondrial; falcon noted this is a prediction rather than direct microscopy/proteomics evidence.

"AGXT2L2 (PHYKPL) is described as a **cytosolic enzyme**, consistent with **TargetP** and **Psort II** predictions cited in the biochemical characterization paper."

Suggested Experiments

Experiment: Crystallize human PHYKPL with substrate analogs and PLP to determine 3D structure

Hypothesis: Structure will reveal unique active site features explaining substrate specificity for phosphohydroxylysine

Type: Structural biology

Experiment: Compare metabolite profiles in PHYKPL knockout vs wild-type cells under high collagen turnover conditions

Hypothesis: Loss of PHYKPL will lead to accumulation of 5-phosphohydroxylysine and reveal alternative catabolic routes

Type: Metabolomics

Experiment: Identify PHYKPL protein interaction partners in mitochondrial matrix using proximity labeling (BioID)

Hypothesis: PHYKPL forms complexes with other lysine catabolic enzymes for efficient substrate channeling

Type: Proteomics

Experiment: Analyze PHYKPL expression across tissues and developmental stages using single-cell RNA-seq

Hypothesis: PHYKPL expression correlates with tissue collagen content and turnover rates

Type: Expression profiling

Experiment: Use 13C-labeled hydroxylysine to trace flux through PHYKPL pathway in various cell types

Hypothesis: Flux through PHYKPL varies with cell type and metabolic state

Type: Metabolic flux analysis

Deep Research

Falcon

(PHYKPL-deep-research-falcon.md)

this with annotations you find in gene/protein databases, but these can be outdated or inaccurate. Falcon Edison Scientific Literature 14 citations 2 artifacts 2026-05-29T20:00:55.812903

Edison artifact artifact-00 (text/markdown)

## Context ID: pqac-00000012 The kinetic parameters for AGXT2L2 (5-phosphohydroxy-L-lysine phospholyase) are reported in Figure 4 and the corresponding text on (image/png)

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research report: Human PHYKPL (AGXT2L2; UniProt Q8IUZ5)

Executive summary

PHYKPL (synonym AGXT2L2) encodes a pyridoxal-5′-phosphate (PLP)-dependent ammoniophospholyase whose primary, experimentally demonstrated function is to catalyze elimination of 5-phosphohydroxy-L-lysine (5PHyl) to yield 2-aminoadipate semialdehyde (2-AASA), inorganic phosphate (Pi), and ammonia (NH3). This reaction places PHYKPL in a defined pathway for hydroxylysine catabolism downstream of a 5-hydroxylysine kinase (AGPHD1/HYKK). The best-supported Mendelian phenotype link is phosphohydroxylysinuria, consistent with expectation that loss of PHYKPL activity increases urinary 5PHyl. Recent (2023–2024) literature retrieved here contains only indirect mentions (e.g., homology-based discussion in PLP enzyme/heme metabolism contexts), while disease-association resources (Open Targets) continue to track a strong association to phosphohydroxylysinuria and weaker GWAS-derived associations to several complex diseases. (veigadacunha2012molecularidentificationof pages 6-7, veigadacunha2012molecularidentificationof pages 1-2, OpenTargets Search: -PHYKPL,AGXT2L2, key2024clppnulleukaryoteswitha pages 2-4)

1) Key concepts and definitions (current understanding)

1.1 Gene/protein identity and nomenclature

The UniProt entry Q8IUZ5 corresponds to human PHYKPL, also known as AGXT2L2, annotated as 5-phosphohydroxy-L-lysine phospho-lyase (EC 4.2.3.134). Primary biochemistry establishing this identity comes from recombinant expression and activity assays showing specific cleavage of 5PHyl to Pi, NH3 and 2-AASA. (veigadacunha2012molecularidentificationof pages 1-2, veigadacunha2012molecularidentificationof pages 3-4)

1.2 Enzyme class: PLP-dependent “ammoniophospholyase”

Although PHYKPL/AGXT2L2 is homologous to class-III PLP-dependent aminotransferases, the experimentally supported activity is a PLP-dependent elimination (phospho-lyase) rather than transamination. In the defining biochemical study, AGXT2L2 was purified as a yellow PLP-binding protein, consistent with PLP cofactor association, and aminotransferase activity was not detected in the authors’ assays for the AGXT2L1/2 paralogs. (veigadacunha2012molecularidentificationof pages 3-4, veigadacunha2012molecularidentificationof pages 7-8)

1.3 Reaction and pathway placement: hydroxylysine catabolism

PHYKPL catalyzes:

5-phosphohydroxy-L-lysine (5PHyl) → 2-aminoadipate semialdehyde (2-AASA) + Pi + NH3

This reaction functions downstream of a kinase (AGPHD1) that phosphorylates 5-hydroxy-L-lysine to create the 5PHyl substrate, establishing a two-step metabolic route for hydroxylysine degradation. (veigadacunha2012molecularidentificationof pages 1-2, veigadacunha2012molecularidentificationof pages 5-6)

2) Functional biochemistry: substrate specificity and kinetics

2.1 Experimental evidence for substrate specificity

The key functional assignment is supported by direct in vitro assays using recombinant human AGXT2L2, where production of Pi, NH3, and the aldehyde product 2-AASA (measured as a derivatized product by RP-HPLC) was observed after incubation with 5PHyl. (veigadacunha2012molecularidentificationof pages 3-4)

The defining study also reports that, under their conditions, the AGXT2L1/2 paralogs did not show detectable aminotransferase activity in the specific assays performed, reinforcing that the physiologically relevant activity for AGXT2L2 is the phospho-lyase reaction. (veigadacunha2012molecularidentificationof pages 7-8)

2.2 Quantitative kinetics and inhibition

Kinetic parameters reported for recombinant human AGXT2L2 acting on 5PHyl include:
- Km(5PHyl) = 16.9 ± 4.5 μM
- Vmax = 256 ± 15 nmol·min⁻¹·mg⁻¹ protein

The saturation behavior shows substrate inhibition at higher 5PHyl concentrations (notably above ~100 μM in the saturation curve) and inhibition by inorganic phosphate, with the highest tested Pi reducing activity by ~50% in the reported ammonia-production readout. (veigadacunha2012molecularidentificationof pages 6-7, veigadacunha2012molecularidentificationof media 1ef79764)

3) Cellular/subcellular localization and expression

3.1 Subcellular localization

AGXT2L2 (PHYKPL) is described as a cytosolic enzyme, consistent with TargetP and Psort II predictions cited in the biochemical characterization paper. (veigadacunha2012molecularidentificationof pages 6-7)

3.2 Tissue expression and physiological context

Direct quantitative tissue expression profiling for PHYKPL/AGXT2L2 was not available in the retrieved full-text excerpts. However:
- The AGXT2L2 coding sequence was cloned from human kidney cDNA, consistent with kidney expression. (veigadacunha2012molecularidentificationof pages 2-3)
- The same study notes that 5PHyl is a normal constituent of adult brain, which motivates concern that loss of AGXT2L2 could cause brain accumulation of 5PHyl with neurological effects (see disease section). (veigadacunha2012molecularidentificationof pages 6-7)

4) Disease associations, applications, and real-world implementations

4.1 Mendelian/metabolic disease: phosphohydroxylysinuria

The defining 2012 biochemical study predicts that mutations in AGXT2L2 should result in increased urinary 5PHyl excretion, and notes that only two unrelated patients with phosphohydroxylysinuria and neurological abnormalities had been reported in earlier literature at that time. (veigadacunha2012molecularidentificationof pages 6-7)

From an implementation perspective, this implies a clinically actionable diagnostic logic: urinary 5PHyl elevation is a plausible biochemical marker indicating deficiency in the AGPHD1→AGXT2L2 hydroxylysine-catabolic pathway, and the molecular identification of AGXT2L2 enables confirmatory genetic diagnosis. (veigadacunha2012molecularidentificationof pages 6-7, veigadacunha2012molecularidentificationof pages 1-2)

4.2 Complex disease genetics (database-level evidence)

Open Targets reports PHYKPL associations including:
- Phosphohydroxylysinuria (highest score among listed diseases; composite score ~0.569)
- GWAS-derived genetic associations to ulcerative colitis, dental caries, glomerulonephritis, and primary thrombocytopenia (scores roughly ~0.27–0.32 in the displayed output)

These complex-disease links are primarily genetic association signals rather than established mechanistic disease roles for the enzyme. (OpenTargets Search: -PHYKPL,AGXT2L2)

4.3 Clinical trials and therapeutics

A clinical-trials search for “PHYKPL OR AGXT2L2” did not yield relevant interventional trials targeting this gene/protein in the retrieved results. (OpenTargets Search: -PHYKPL,AGXT2L2)

5) Recent developments and latest research (prioritizing 2023–2024)

5.1 2024: broader PLP-enzyme/mitochondrial metabolism discussion

A 2024 study focused on CLPP/CLPX biology and metabolomics mentions PHYKPL as one of several PLP-dependent proteins with sequence homology to bacterial HemL (glutamate-1-semialdehyde aminomutase), in the context of hypothesized alternative δ-aminolevulinic acid (δALA) production routes. This is not new experimental evidence about PHYKPL’s physiological function, but illustrates how PHYKPL appears in comparative discussions of PLP-enzyme repertoires. (key2024clppnulleukaryoteswitha pages 2-4)

5.2 2023–2024 gaps in the retrievable corpus

Within the documents retrievable here, there were no 2023–2024 primary studies that provided new direct biochemical characterization, localization by microscopy/proteomics, or targeted functional genetics for human PHYKPL beyond the established 2012 biochemistry and database association updates. Therefore, current best practice for functional annotation remains anchored in the 2012 enzymology while monitoring genetic databases and emerging omics studies for new physiology or disease insights. (key2024clppnulleukaryoteswitha pages 2-4, OpenTargets Search: -PHYKPL,AGXT2L2)

6) Expert interpretation and analysis (authoritative sources)

The most authoritative mechanistic evidence is the enzymology work establishing that vertebrate genomes encode AGXT2L1/AGXT2L2 as enzymes acting on phosphorylated terminal amines, and that AGXT2L2 specifically is the 5PHyl phospho-lyase. The authors explicitly connect this to explain rare neurometabolic phenotypes, and they propose AGPHD1 and AGXT2L2 as likely mutated enzymes in 5-hydroxylysinuria and phosphohydroxylysinuria. (veigadacunha2012molecularidentificationof pages 1-2)

A key interpretive nuance is that phylogenetic homology to aminotransferases led earlier studies to test AGXT2L2 as a transaminase; e.g., a 2009 recombinant expression study screened AGXT2L2 for glyoxylate transamination but did not find glycine production under their conditions, and noted that reaction preferences were still unclear at that time. The subsequent 2012 work resolved the principal activity as a phospho-lyase, illustrating how enzyme superfamily membership can mislead functional inference without substrate-guided biochemistry. (donini2009recombinantproductionof pages 10-12, veigadacunha2012molecularidentificationof pages 1-2)

7) Key statistics and data points from studies

Enzyme kinetics (recombinant human PHYKPL/AGXT2L2): Km(5PHyl) 16.9 ± 4.5 μM; Vmax 256 ± 15 nmol·min⁻¹·mg⁻¹; substrate inhibition above ~100 μM 5PHyl. (veigadacunha2012molecularidentificationof media 1ef79764, veigadacunha2012molecularidentificationof pages 6-7)
Disease case count noted in foundational report: phosphohydroxylysinuria had been reported in two unrelated patients with neurological abnormalities (as summarized in the 2012 study). (veigadacunha2012molecularidentificationof pages 6-7)
Database-level disease association scores (Open Targets): phosphohydroxylysinuria score ~0.569; ulcerative colitis ~0.319; dental caries ~0.310; glomerulonephritis ~0.296; primary thrombocytopenia ~0.269 (as displayed). (OpenTargets Search: -PHYKPL,AGXT2L2)

Evidence map (summary table)

Aspect	Finding	Key evidence/source (paper and year)	Notes/limitations
Gene/protein identity	Human PHYKPL / AGXT2L2 encodes 5-phosphohydroxy-L-lysine phospho-lyase; a PLP-dependent enzyme matching UniProt Q8IUZ5	Veiga-da-Cunha et al., J Biol Chem (2012) (veigadacunha2012molecularidentificationof pages 1-2, veigadacunha2012molecularidentificationof pages 3-4)	Identity is well matched to the UniProt entry; older literature often used the synonym AGXT2L2 rather than PHYKPL
Enzymatic reaction	Catalyzes breakdown of 5-phosphohydroxy-L-lysine (5PHyl) to 2-aminoadipate semialdehyde (2-AASA) + NH3 + Pi	Veiga-da-Cunha et al. (2012) (veigadacunha2012molecularidentificationof pages 6-7, veigadacunha2012molecularidentificationof pages 1-2, veigadacunha2012molecularidentificationof pages 3-4)	Experimentally demonstrated with recombinant human enzyme; this is the primary functional annotation
Cofactor / enzyme class	Pyridoxal phosphate (PLP)-dependent ammoniophospholyase within the class-III aminotransferase-fold family	Veiga-da-Cunha et al. (2012) (veigadacunha2012molecularidentificationof pages 1-2, veigadacunha2012molecularidentificationof pages 3-4)	Despite homology to aminotransferases, no aminotransferase activity was detected in the reported assays
Key kinetics	Km = 16.9 ± 4.5 µM for 5PHyl; Vmax = 256 ± 15 nmol·min⁻¹·mg⁻¹ protein	Veiga-da-Cunha et al. (2012), Figure 4/text (veigadacunha2012molecularidentificationof pages 6-7, veigadacunha2012molecularidentificationof media 1ef79764)	Saturation curve showed substrate inhibition above ~100 µM 5PHyl
Product / inhibitor behavior	Reaction yields stoichiometric phosphate, ammonia, and 2-AASA; inorganic phosphate inhibits activity, with ~50% reduction at the highest tested Pi (2–10 mM range tested)	Veiga-da-Cunha et al. (2012) (veigadacunha2012molecularidentificationof pages 6-7)	Supports assignment as a phospho-lyase; inhibition data are biochemical, not in vivo physiology
Evidence type	Function established by recombinant human protein expression, purification, and in vitro enzyme assays measuring Pi, NH3, and 2-AASA formation	Veiga-da-Cunha et al. (2012) (veigadacunha2012molecularidentificationof pages 3-4)	Strong primary-biochemistry evidence; human cellular knockout/knockdown studies remain limited in retrieved sources
Pathway context	Functions in hydroxylysine catabolism downstream of AGPHD1/HYKK (5-hydroxy-L-lysine kinase), converting phosphorylated hydroxylysine to 2-AASA	Veiga-da-Cunha et al. (2012); Van Schaftingen et al. (2015) (veigadacunha2012molecularidentificationof pages 5-6, veigadacunha2012molecularidentificationof pages 1-2)	Pathway placement is well supported biochemically, but broader physiological flux regulation in humans is still sparsely characterized
Localization	Reported/predicted as cytosolic based on bioinformatic localization analyses	Veiga-da-Cunha et al. (2012) (veigadacunha2012molecularidentificationof pages 6-7, veigadacunha2012molecularidentificationof pages 5-6)	Retrieved evidence indicates prediction rather than extensive direct microscopy/proteomics confirmation
Disease / Mendelian link	Proposed disease gene for phosphohydroxylysinuria; authors expected AGXT2L2 defects to elevate urinary 5PHyl	Veiga-da-Cunha et al. (2012); Open Targets summary (veigadacunha2012molecularidentificationof pages 6-7, OpenTargets Search: -PHYKPL,AGXT2L2)	A 2013 JIMD mutation paper was identified in search results but not retrievable here; therefore this row relies on the 2012 primary paper plus database-level association evidence
Other disease associations	Open Targets lists additional associations with ulcerative colitis, dental caries, glomerulonephritis, and primary thrombocytopenia besides phosphohydroxylysinuria	Open Targets Platform query (accessed via tool context) (OpenTargets Search: -PHYKPL,AGXT2L2)	These are mostly genetic-association/database links, not direct mechanistic validation specific to PHYKPL biology
Recent 2023–2024 mentions	2024 literature mentions PHYKPL mainly as a PLP-enzyme homolog in comparative/heme-metabolism discussion rather than providing new direct functional data	Key et al., Biomolecules (2024) (key2024clppnulleukaryoteswitha pages 2-4)	No major 2023–2024 primary studies with new biochemical or localization data for human PHYKPL were retrieved
Current annotation confidence	High confidence for core enzymatic function; moderate-to-low confidence for localization detail, tissue physiology, and broader disease relevance	Synthesis of primary biochemistry and database evidence (veigadacunha2012molecularidentificationof pages 6-7, veigadacunha2012molecularidentificationof pages 1-2, OpenTargets Search: -PHYKPL,AGXT2L2, veigadacunha2012molecularidentificationof media 1ef79764)	Literature on this specific human protein remains limited compared with better-studied metabolic enzymes

Table: This table summarizes the core functional annotation of human PHYKPL/AGXT2L2, including its experimentally demonstrated phospho-lyase reaction, kinetic parameters, pathway placement, localization, and disease links. It is useful as a compact evidence map separating strong biochemical findings from more tentative disease-association data.

URLs and publication dates (key sources)

Veiga-da-Cunha M. et al. “Molecular Identification of Hydroxylysine Kinase and of Ammoniophospholyases Acting on 5-Phosphohydroxy-L-lysine and Phosphoethanolamine.” Journal of Biological Chemistry (Mar 2012). https://doi.org/10.1074/jbc.M111.323485 (veigadacunha2012molecularidentificationof pages 1-2, veigadacunha2012molecularidentificationof media 1ef79764)
Key J. et al. “CLPP-Null Eukaryotes with Excess Heme Biosynthesis Show Reduced L-arginine Levels, Probably via CLPX-Mediated OAT Activation.” Biomolecules (Feb 2024). https://doi.org/10.3390/biom14020241 (key2024clppnulleukaryoteswitha pages 2-4)
Open Targets Platform disease-target associations for PHYKPL (queried via tool; platform described in Nucleic Acids Research, 2025). https://platform.opentargets.org/ (OpenTargets Search: -PHYKPL,AGXT2L2)

Limitations of this review

A key limitation is that an apparently central Mendelian genetics paper (“Mutations in the AGXT2L2 gene cause phosphohydroxylysinuria,” J Inherit Metab Dis, 2013) was identified by the search system but could not be retrieved in full text in this run, limiting direct quotation of variant types, patient counts, and clinical phenotypes beyond what is summarized in the 2012 biochemical report and Open Targets. (veigadacunha2012molecularidentificationof pages 6-7, OpenTargets Search: -PHYKPL,AGXT2L2)

References

(veigadacunha2012molecularidentificationof pages 6-7): Maria Veiga-da-Cunha, Farah Hadi, Thomas Balligand, Vincent Stroobant, and Emile Van Schaftingen. Molecular identification of hydroxylysine kinase and of ammoniophospholyases acting on 5-phosphohydroxy-l-lysine and phosphoethanolamine. Journal of Biological Chemistry, 287:7246-7255, Mar 2012. URL: https://doi.org/10.1074/jbc.m111.323485, doi:10.1074/jbc.m111.323485. This article has 71 citations and is from a domain leading peer-reviewed journal.
(veigadacunha2012molecularidentificationof pages 1-2): Maria Veiga-da-Cunha, Farah Hadi, Thomas Balligand, Vincent Stroobant, and Emile Van Schaftingen. Molecular identification of hydroxylysine kinase and of ammoniophospholyases acting on 5-phosphohydroxy-l-lysine and phosphoethanolamine. Journal of Biological Chemistry, 287:7246-7255, Mar 2012. URL: https://doi.org/10.1074/jbc.m111.323485, doi:10.1074/jbc.m111.323485. This article has 71 citations and is from a domain leading peer-reviewed journal.
(OpenTargets Search: -PHYKPL,AGXT2L2): Open Targets Query (-PHYKPL,AGXT2L2, 5 results). Buniello, A. et al. (2025). Open Targets Platform: facilitating therapeutic hypotheses building in drug discovery. Nucleic Acids Research.
(key2024clppnulleukaryoteswitha pages 2-4): Jana Key, Suzana Gispert, Arvind Reddy Kandi, Daniela Heinz, Andrea Hamann, Heinz D. Osiewacz, David Meierhofer, and Georg Auburger. Clpp-null eukaryotes with excess heme biosynthesis show reduced l-arginine levels, probably via clpx-mediated oat activation. Biomolecules, 14:241, Feb 2024. URL: https://doi.org/10.3390/biom14020241, doi:10.3390/biom14020241. This article has 2 citations.
(veigadacunha2012molecularidentificationof pages 3-4): Maria Veiga-da-Cunha, Farah Hadi, Thomas Balligand, Vincent Stroobant, and Emile Van Schaftingen. Molecular identification of hydroxylysine kinase and of ammoniophospholyases acting on 5-phosphohydroxy-l-lysine and phosphoethanolamine. Journal of Biological Chemistry, 287:7246-7255, Mar 2012. URL: https://doi.org/10.1074/jbc.m111.323485, doi:10.1074/jbc.m111.323485. This article has 71 citations and is from a domain leading peer-reviewed journal.
(veigadacunha2012molecularidentificationof pages 7-8): Maria Veiga-da-Cunha, Farah Hadi, Thomas Balligand, Vincent Stroobant, and Emile Van Schaftingen. Molecular identification of hydroxylysine kinase and of ammoniophospholyases acting on 5-phosphohydroxy-l-lysine and phosphoethanolamine. Journal of Biological Chemistry, 287:7246-7255, Mar 2012. URL: https://doi.org/10.1074/jbc.m111.323485, doi:10.1074/jbc.m111.323485. This article has 71 citations and is from a domain leading peer-reviewed journal.
(veigadacunha2012molecularidentificationof pages 5-6): Maria Veiga-da-Cunha, Farah Hadi, Thomas Balligand, Vincent Stroobant, and Emile Van Schaftingen. Molecular identification of hydroxylysine kinase and of ammoniophospholyases acting on 5-phosphohydroxy-l-lysine and phosphoethanolamine. Journal of Biological Chemistry, 287:7246-7255, Mar 2012. URL: https://doi.org/10.1074/jbc.m111.323485, doi:10.1074/jbc.m111.323485. This article has 71 citations and is from a domain leading peer-reviewed journal.
(veigadacunha2012molecularidentificationof media 1ef79764): Maria Veiga-da-Cunha, Farah Hadi, Thomas Balligand, Vincent Stroobant, and Emile Van Schaftingen. Molecular identification of hydroxylysine kinase and of ammoniophospholyases acting on 5-phosphohydroxy-l-lysine and phosphoethanolamine. Journal of Biological Chemistry, 287:7246-7255, Mar 2012. URL: https://doi.org/10.1074/jbc.m111.323485, doi:10.1074/jbc.m111.323485. This article has 71 citations and is from a domain leading peer-reviewed journal.
(veigadacunha2012molecularidentificationof pages 2-3): Maria Veiga-da-Cunha, Farah Hadi, Thomas Balligand, Vincent Stroobant, and Emile Van Schaftingen. Molecular identification of hydroxylysine kinase and of ammoniophospholyases acting on 5-phosphohydroxy-l-lysine and phosphoethanolamine. Journal of Biological Chemistry, 287:7246-7255, Mar 2012. URL: https://doi.org/10.1074/jbc.m111.323485, doi:10.1074/jbc.m111.323485. This article has 71 citations and is from a domain leading peer-reviewed journal.
(donini2009recombinantproductionof pages 10-12): Stefano Donini, Manuela Ferrari, Chiara Fedeli, Marco Faini, Ilaria Lamberto, Ada Serena Marletta, Lara Mellini, Michela Panini, Riccardo Percudani, Loredano Pollegioni, Laura Caldinelli, Stefania Petrucco, and Alessio Peracchi. Recombinant production of eight human cytosolic aminotransferases and assessment of their potential involvement in glyoxylate metabolism. The Biochemical journal, 422 2:265-72, Sep 2009. URL: https://doi.org/10.1042/bj20090748, doi:10.1042/bj20090748. This article has 38 citations.

Artifacts

Edison artifact artifact-00

Citations

veigadacunha2012molecularidentificationof pages 3-4
veigadacunha2012molecularidentificationof pages 7-8
veigadacunha2012molecularidentificationof pages 6-7
veigadacunha2012molecularidentificationof pages 2-3
key2024clppnulleukaryoteswitha pages 2-4
veigadacunha2012molecularidentificationof pages 1-2
veigadacunha2012molecularidentificationof pages 5-6
donini2009recombinantproductionof pages 10-12
https://doi.org/10.1074/jbc.M111.323485
https://doi.org/10.3390/biom14020241
https://platform.opentargets.org/
https://doi.org/10.1074/jbc.m111.323485,
https://doi.org/10.3390/biom14020241,
https://doi.org/10.1042/bj20090748,

Lite

(PHYKPL-deep-research-lite.md)

PHYKPL (5-Phosphohydroxylysine Phospholyase): Enzymatic Function Lite

PHYKPL (5-Phosphohydroxylysine Phospholyase): Enzymatic Function

PHYKPL (formerly AGXT2L2) encodes a pyridoxal-phosphate (PLP)–dependent enzyme that catalyzes the breakdown of 5-phosphohydroxylysine. Biochemical studies show that the enzyme (EC 4.2.3.134) cleaves (5R)-5-phosphonooxy-L-lysine with water to yield 2-amino-6-oxohexanoate (2-aminoadipate semialdehyde), ammonia, and inorganic phosphate[1][2]. In other words, PHYKPL’s product converts phosphorylated hydroxylysine into ammonia, phosphate and a semialdehyde (2-aminoadipate semialdehyde)[2][3]. This activity was first demonstrated by Veiga-da-Cunha et al. (2012), who produced recombinant human PHYKPL and showed it carried out exactly this reaction[3]. The enzyme is known as an ammoniophospholyase (it breaks a C–N bond releasing NH_4^+), despite its sequence similarity to aminotransferases. PHYKPL belongs to the class-III aminotransferase superfamily (the “alanine:glyoxylate aminotransferase” family) and binds PLP as cofactor[4][2]. (Sequence databases list domains IPR005814 etc., consistent with aminotransferases[5].) In line with this, UniProt/GeneCards note “pyridoxal-phosphate-dependent breakdown” of the substrate[2]. In sum, the gene product is a mitochondrial PLP-dependent lyase that catabolizes 5-phosphohydroxylysine. It localizes to the mitochondrial matrix (per prediction and antibody data[6][7]) and is synthesized from a nuclear gene transcript.

Role in Cellular and Biological Processes

PHYKPL serves at a novel juncture between collagen turnover and amino acid metabolism. Specifically, 5-hydroxylysine is an abundant collagen modification; when collagen is degraded, free 5-hydroxylysine can be phosphorylated by a dedicated kinase (HYKK, also called HYKK/AGPHD1) and then cleaved by PHYKPL[3][8]. In other words, 5-phosphohydroxylysine is a breakdown product of collagen. In mitochondria, PHYKPL (as a homotetramer) hydrolyzes this collagen-derived metabolite to feed into the lysine catabolic pathway[8]. The 2-aminoadipate semialdehyde product is a known intermediate in lysine degradation, ultimately leading to acetyl-CoA via the saccharopine/antiquitin route. Thus, PHYKPL connects extracellular matrix breakdown to energy metabolism: it helps dispose of hydroxylysine residues liberated from collagen, converting them into form that enters central metabolism. This link places PHYKPL activity in the broader context of amino-acid catabolism and collagen degradation[8]. The enzyme thereby prevents accumulation of phospho-hydroxylysine and recovers nitrogen and carbon from modified lysine residues. Consistent with this metabolic role, pathologic deficiency of PHYKPL (due to loss-of-function mutations) causes phosphohydroxylysinuria – a condition defined by elevated urinary 5-phosphohydroxylysine[6][9]. Interestingly, this biochemical phenotype appears to have no other apparent clinical manifestations[9], suggesting that the pathway is not essential for survival but is a normal part of amino acid recycling. In sum, PHYKPL’s core function is enzymatic: a PLP-dependent lyase in mitochondria that cleaves a collagen-related metabolite, linking protein (collagen) turnover to lysine/amino acid catabolism[8][3].

Gaps in Current Knowledge

Physiological context: It remains unclear under what conditions 5-phosphohydroxylysine is generated in vivo. While collagen degradation would release 5-hydroxylysine residues, the regulation and flux of this pathway under normal physiology or stress are poorly understood.
Significance of the pathway: Given that patients lacking PHYKPL activity (phosphohydroxylysinuria) show no clinical symptoms beyond metabolite buildup[9], the biological importance of this catabolic route is uncertain. It’s possible redundant disposal routes for hydroxylysine exist, or that any toxicity of accumulated metabolite is minimal.
Enzyme specifics: The structure, substrate specificity and kinetics of human PHYKPL have not been extensively characterized. The enzyme appears related to aminotransferases, yet its lyase activity is unusual; details of its active site and mechanism are unknown.
Regulation and expression: Little is known about how PHYKPL expression or activity is regulated. Tissue-specific and developmental expression patterns have not been fully mapped. (Some expression data hint at modest expression in brain and endocrine tissues, but no clear “tissue of interest” is established.)
Mitochondrial targeting: The mechanism of mitochondrial import of PHYKPL (e.g. targeting sequence) has not been studied.
Evolutionary aspects: Homologs of PHYKPL exist across vertebrates (and even in organisms like Drosophila[10]), but the evolutionary origin of this enzyme and its pathway is not well explored. It’s unclear how conserved hydroxylysine catabolism is across species.
Interacting partners: There is no information on whether PHYKPL forms complexes or is regulated by protein–protein interactions in mitochondria.

Questions for an Expert

Substrate origin: What is the main source of 5-phosphohydroxylysine in human metabolism? Is it derived solely from collagen breakdown, or might there be other pathways generating free hydroxylysine that needs disposal?
Redundancy: Are there alternative enzymes or pathways that can metabolize 5-phosphohydroxylysine if PHYKPL is absent? (The benign nature of phosphohydroxylysinuria hints at possible redundancy.)
Structure/function: What is known about the three-dimensional structure or key catalytic residues of PHYKPL? Has any structure been solved, or are there informative homology models?
Regulation: How is PHYKPL expression controlled? Are there known transcription factors or signaling pathways that up/downregulate this gene under specific conditions (e.g. tissue remodeling, vitamin B6 status)?
Tissue specificity: In which cell types or tissues is PHYKPL most active or abundant? Does it vary with development or metabolism?
Physiological role: Despite no overt disease, could PHYKPL have subtle roles (e.g. in nitrogen balance, redox state, or disease susceptibility)? Could there be unrecognized phenotypes in knockout models or under stress?
Evolution: What do comparative genomics and phylogeny tell us about PHYKPL’s origins? Are bacterial ancestors of vertebrates known to have related enzymes, and what function do they serve?
Potential interactions: Does PHYKPL associate with other mitochondrial enzymes (e.g. lysine catabolic enzymes) in a complex? Could it be coordinated with upstream (kinase) or downstream (dehydrogenase) steps?

Experimental Approaches to Elucidate Function

Biochemical assays: Express and purify human PHYKPL for in vitro enzymology. Measure its activity with synthetic 5-phosphohydroxylysine (and analogs) to determine kinetic parameters (Km, Vmax) and cofactor requirements (confirm PLP dependence)[2][1]. Test whether it can act on related substrates (e.g. different phosphoamino acids) to define specificity.
Cellular knockdown/knockout: Use CRISPR or siRNA in cultured human cells to disrupt PHYKPL. Then use targeted metabolomics (mass spectrometry) to detect accumulation of 5-phosphohydroxylysine or its metabolites. Compare to wild-type cells under normal and stress conditions (e.g. with added collagen fragments or excess lysine).
Animal models: Generate a PHYKPL-deficient mouse or zebrafish. Analyze urine and tissues for phosphohydroxylysine accumulation. Monitor for subtle phenotypes (growth, metabolism, response to high-protein diet, etc.). This could reveal in vivo roles or compensation by other pathways.
Metabolic tracing: Feed cells or model organisms isotopically labeled 5-hydroxylysine or collagen. Trace labeled atoms through phosphorylation by HYKK and cleavage by PHYKPL. This would confirm flux through this pathway and quantify its contribution to overall lysine catabolism.
Structural biology: Crystallize PHYKPL (or cryo-EM) to solve its structure. Identify active-site residues and compare to classical aminotransferases. Mutagenesis of key residues (e.g. PLP-binding lysine) could validate mechanism.
Expression profiling: Perform RNA/protein expression screens across tissues, development stages, or disease states. This could identify when and where PHYKPL is upregulated. Single-cell RNA-seq could pinpoint specific cell types expressing it.
Interaction studies: Use co-immunoprecipitation or proximity labeling (BioID) to see if PHYKPL interacts with other mitochondrial proteins or metabolic enzymes. Identifying partners might reveal regulatory mechanisms.
Knock-in variants: Introduce human PHYKPL mutations (e.g. those found in phosphohydroxylysinuria patients) into cell lines to study their effects on enzyme function and cell physiology.
Cofactor dependence: Test how cellular pyridoxal phosphate (vitamin B6) levels affect PHYKPL activity in cells, since it requires PLP[2]. Vitamin B6 deficiency might impair the enzyme and reveal latent phenotypes.

These studies together would fill gaps about PHYKPL’s substrate sources, regulation, and physiological importance. By combining biochemical, genetic, and analytical approaches, one could map the precise role of PHYKPL in amino acid and collagen metabolism and determine why its deficiency has so little clinical impact.

Sources: Information synthesized from UniProt/RefSeq summaries and enzymology references[2][6][1][3][8][9], as well as curated pathway databases.

[1] ENZYME - 4.2.3.134 5-phosphooxy-L-lysine phospho-lyase

https://enzyme.expasy.org/EC/4.2.3.134

[2] PHYKPL Gene - GeneCards | AT2L2 Protein | AT2L2 Antibody

https://www.genecards.org/cgi-bin/carddisp.pl?gene=PHYKPL

[3] [4] Molecular identification of hydroxylysine kinase and of ammoniophospholyases acting on 5-phosphohydroxy-L-lysine and phosphoethanolamine - PubMed

https://pubmed.ncbi.nlm.nih.gov/22241472/

[5] Phykpl MGI Mouse Gene Detail - MGI:1920197 - 5-phosphohydroxy-L-lysine phospholyase

https://www.informatics.jax.org/marker/MGI:1920197

[6] PHYKPL 5-phosphohydroxy-L-lysine phospho-lyase AGXT2L2 PHLU | Sigma-Aldrich

https://www.sigmaaldrich.com/US/en/genes/phykpl?srsltid=AfmBOorRF7gEJNoWdGFX-SP7zfpVqvtuxiz91bgf-ivl-X3pIV6ehhC8

[7] PHYKPL protein expression summary - The Human Protein Atlas

https://www.proteinatlas.org/ENSG00000175309-PHYKPL

[8] [10] Reactome | PXLP-K278-PHYKPL tetramer hydrolyses 5PHL

https://reactome.org/content/detail/R-HSA-5696408

[9] Phosphohydroxylysinuria - MalaCards

https://www.malacards.org/card/phosphohydroxylysinuria?search=PHYKPL

📚 Additional Documentation

Falcon Research

(PHYKPL-falcon-research.md)

Falcon Research Report: PHYKPL (human)

Generated using FutureHouse Falcon API

UniProt ID: Q8IUZ5
Directory alias: PHYKPL

Question: You are a molecular biologist and gene annotation expert conducting comprehensive research to support GO annotation curation.

Provide detailed, well-cited information focusing on:
1. Gene function and molecular mechanisms
2. Cellular localization and subcellular components
3. Biological processes involvement
4. Disease associations and phenotypes
5. Protein domains and structural features
6. Expression patterns and regulation
7. Evolutionary conservation
8. Key experimental evidence and literature

Format as a comprehensive research report with citations suitable for Gene Ontology annotation curation.

Research the Homo sapiens gene PHYKPL.

=== UNIPROT METADATA ===
UniProt ID: Q8IUZ5
Entry Name: AT2L2_HUMAN
Gene Name: PHYKPL
Protein Name: 5-phosphohydroxy-L-lysine phospho-lyase
EC Number: 4.2.3.134
Organism: Homo sapiens (Human)
NCBI Taxonomy ID: 9606
Function: Catalyzes the pyridoxal-phosphate-dependent breakdown of 5- phosphohydroxy-L-lysine, converting it to ammonia, inorganic phosphate and 2-aminoadipate semialdehyde.
Subcellular Location: Mitochondrion
Subunit: Homotetramer.
======================

Provide a comprehensive report covering function, localization, processes, domains, disease associations, expression, conservation, and relevant GO terms.

IMPORTANT: Focus specifically on the gene identified by the metadata above, particularly the UniProt ID, locus tag, and protein description if provided.

Sometimes different genes in the same organism have the same name. In this case, the gene being reviewed is the one identified by the metadata above.

The following report provides a comprehensive overview of the Homo sapiens gene PHYKPL (UniProt: Q8IUZ5, Entry Name AT2L2_HUMAN), which encodes the enzyme 5‐phosphohydroxy‐L‐lysine phospho‐lyase. This enzyme catalyzes the pyridoxal phosphate (PLP)–dependent conversion of 5‐phosphohydroxy‐L‐lysine to ammonia, inorganic phosphate, and 2‐aminoadipate semialdehyde, a key step in lysine catabolism. In addition to its central role in amino acid metabolism, PHYKPL has been linked to alterations in erythrocyte and reticulocyte phenotypes as well as to the rare metabolic disorder phosphohydroxylysinuria. The following sections detail its gene function and molecular mechanisms, cellular localization, involvement in biological processes, disease associations, protein domains and structural features, expression patterns and regulation, evolutionary conservation, and key experimental evidence that underpins its functional annotation.

Abstract
PHYKPL is a mitochondrial enzyme that mediates a crucial step in lysine degradation through a PLP‐dependent phospho‐lyase reaction. Its activity impacts red blood cell development by modulating reticulocyte and erythrocyte parameters, and genetic data strongly associate perturbations in PHYKPL with phosphohydroxylysinuria. The enzyme is localized in the mitochondrial matrix as a homotetramer and contains domains characteristic of PLP‐dependent enzymes. Expression studies have revealed tissue‐specific regulation, informing its potential roles in metabolic tissues, while evolutionary analyses demonstrate strong conservation among mammals. Here, we collate and synthesize high‐quality genetic, biochemical, and bioinformatic evidence to support comprehensive Gene Ontology (GO) annotation for PHYKPL.

Introduction
PHYKPL (5‐phosphohydroxy‐L‐lysine phospho‐lyase) is a human gene whose product functions as a key catalyst in the catabolic degradation of 5‐phosphohydroxy‐L‐lysine, a modified lysine derivative. The enzyme uses pyridoxal 5′‐phosphate (PLP) as an essential cofactor to drive its reaction, converting 5‐phosphohydroxy‐L‐lysine into ammonia, inorganic phosphate, and 2‐aminoadipate semialdehyde, thereby participating in the lysine catabolic pathway (OpenTargets Search: -PHYKPL, liang2019currentadvanceson pages 19-20). In addition to its metabolic role, high‐confidence genetic association studies have linked mutations and missense variants in PHYKPL to altered erythrocyte characteristics such as reticulocyte count and volume, and to the rare disorder phosphohydroxylysinuria (OpenTargets Search: -PHYKPL). Because lysine catabolism is integral to cellular homeostasis, particularly in metabolically active tissues, characterizing the molecular function and regulation of PHYKPL is essential for both basic biology and clinical investigation.
Gene Function and Molecular Mechanisms
PHYKPL encodes the enzyme 5‐phosphohydroxy‐L‐lysine phospho‐lyase, which catalyzes an irreversible reaction in the lysine degradation pathway. The enzyme operates in a manner that is dependent on the pyridoxal 5′‐phosphate (PLP) cofactor, a vitamin B6 derivative known to act as an electrophilic catalyst in numerous enzymatic reactions involving amino acids. In this reaction, the enzyme mediates the breakdown of the phosphorylated derivative of hydroxylysine; by cleaving the substrate, PHYKPL liberates ammonia and inorganic phosphate, while generating 2‐aminoadipate semialdehyde as an intermediate that can be further processed in downstream metabolic pathways (OpenTargets Search: -PHYKPL, liang2019currentadvanceson pages 19-20). The PLP dependency is a critical attribute as it underlies the enzyme’s catalytic mechanism; PLP forms an internal aldimine with a conserved lysine residue within the active site and subsequently is exchanged by the substrate to form a reactive external aldimine intermediate. Such mechanistic details exemplify the characteristic reaction modes of PLP‐dependent enzymes and are supported by structural and biochemical studies on enzymes with similar functions (liang2019currentadvanceson pages 19-20). In addition, the enzyme’s activity in lysine catabolism is evidenced by genetic association studies showing that missense variants within the PHYKPL coding region correlate with variations in erythrocyte parameters, suggesting that its catalytic output affects metabolic processes within hematopoietic cells (OpenTargets Search: -PHYKPL).
Cellular Localization and Subcellular Components
The functional assignment of PHYKPL as a mitochondrial enzyme is supported by both experimental evidence and predictive bioinformatic analyses. According to UniProt metadata and corroborated by literature, PHYKPL localizes to the mitochondrion where it participates in lysine catabolism (OpenTargets Search: -PHYKPL, rosso2008mitochondrialtargetingadaptation pages 8-8). Mitochondrial targeting signals in the N-terminal region facilitate the import of this enzyme into the mitochondrial matrix, where high-energy metabolic processes including amino acid catabolism take place. Once imported into the matrix, PHYKPL assembles as a homotetramer, a quaternary structure that is common among PLP-dependent enzymes enabling efficient substrate channeling and regulation of activity (liang2019currentadvanceson pages 19-20, key2023translationfidelityand pages 34-35). The mitochondrial localization is particularly significant given that mitochondria serve as hubs for essential metabolic pathways, and the positioning of PHYKPL in this compartment ensures optimal integration with other components of amino acid degradation, the tricarboxylic acid cycle, and overall cellular energy production (OpenTargets Search: -PHYKPL, rosso2008mitochondrialtargetingadaptation pages 8-8).
Biological Processes Involvement
The primary biological process associated with PHYKPL is lysine catabolism. By catalyzing the breakdown of 5‐phosphohydroxy‐L‐lysine, PHYKPL contributes directly to the cellular handling and disposal of lysine and its derivatives, thereby influencing amino acid turnover and energy balance (OpenTargets Search: -PHYKPL, liang2019currentadvanceson pages 19-20). Moreover, the modulation of lysine catabolic flux by PHYKPL is indirectly linked to erythropoiesis and red blood cell development. Several genetic studies have revealed associations between variations in PHYKPL and traits such as reticulocyte count, reticulocyte volume, and overall erythrocyte volume (OpenTargets Search: -PHYKPL). These intriguing observations suggest that the metabolic intermediates produced by PHYKPL activity may influence the metabolic environment within hematopoietic tissues, contributing to the regulation of blood cell development. In addition, alterations in lysine degradation may have broader implications for metabolic homeostasis, including potential links to secondary pathways such as heme biosynthesis and the regulation of cellular redox states (OpenTargets Search: -PHYKPL, key2023translationfidelityand pages 34-35).
Disease Associations and Phenotypes
One of the most significant clinical associations for PHYKPL is its strong link to the rare disorder phosphohydroxylysinuria. Patients with phosphohydroxylysinuria exhibit metabolic defects that are believed to stem from dysregulation of lysine catabolism, which in turn leads to abnormal accumulation of the substrate 5‐phosphohydroxy‐L‐lysine or its downstream metabolites (OpenTargets Search: -PHYKPL). In addition to this rare metabolic disorder, genetic association studies have also connected PHYKPL to common erythrocyte-related phenotypes. Specifically, variations in PHYKPL have been correlated with differences in reticulocyte counts and erythrocyte volume, underscoring its potential impact on red blood cell homeostasis and suggesting that even subtle alterations in its function could have measurable hematological consequences (OpenTargets Search: -PHYKPL, zhang2020pofut1isdispensable pages 7-9). These findings provide an informative link between the molecular function of a lysine catabolic enzyme and clinically observable traits in the hematopoietic system, highlighting the importance of precise metabolic regulation in maintaining blood cell integrity.
Protein Domains and Structural Features
The enzymatic activity of PHYKPL is underpinned by its domain architecture and structural organization. As a PLP-dependent enzyme, PHYKPL relies on a catalytic domain that binds the pyridoxal 5′-phosphate cofactor, facilitating the formation of a Schiff base with an active-site lysine residue. This alignment of catalytic residues ensures effective stabilization of reaction intermediates during the cleavage of 5‐phosphohydroxy‐L‐lysine (liang2019currentadvanceson pages 19-20). In addition, biochemical studies of similar PLP-dependent enzymes suggest that such proteins typically assemble as homotetramers, an arrangement that has been confirmed for PHYKPL by both experimental and structural predictions (liang2019currentadvanceson pages 19-20, key2023translationfidelityand pages 34-35). Although high-resolution crystal structures specific to PHYKPL have not been detailed in the context provided, the conservation of key catalytic motifs and its classification in the enzyme commission number EC 4.2.3.134 reinforce its placement within a well‐characterized family of amino acid–modifying enzymes. Moreover, the presence of conserved PLP-binding domains indicates that the catalytic mechanism of PHYKPL follows a reaction paradigm common to other enzymes involved in amino acid metabolism, thereby enabling comparative analyses and extrapolation of structural-functional relationships from homologous systems.
Expression Patterns and Regulation
Expression studies and cis-eQTL analyses have provided insight into the regulation of PHYKPL across different human tissues, particularly in metabolically active organs. Quantitative trait locus mapping in liver and adipose tissues demonstrates that regulatory variation at the PHYKPL locus can significantly alter its transcript levels, with a highly significant cis-eQTL detected in the liver and correlated changes in adipose tissue expression (norheim2017geneticandhormonal pages 26-29). Although the correlations with hepatic triglyceride phenotypes and related metabolic traits are modest, these data imply that PHYKPL expression is subject to tight regulatory control in tissues where amino acid catabolism and energy metabolism are critical. In the context of hematopoiesis, the association between PHYKPL expression and reticulocyte as well as erythrocyte traits further indicates that its regulation may be tuned according to specific metabolic demands in circulating blood cells (OpenTargets Search: -PHYKPL, zhang2020pofut1isdispensable pages 7-9). Overall, the observed tissue-specific expression patterns support a model in which PHYKPL activity is integrated into the broader network of mitochondrial and metabolic regulation, ensuring that lysine degradation is appropriately balanced with other cellular processes.
Evolutionary Conservation
Evolutionary analyses suggest that PHYKPL is an evolutionarily conserved component of lysine metabolism in mammals. The strong conservation of its catalytic domain, particularly the PLP-binding motifs, indicates that the enzyme’s function in lysine degradation has been maintained through purifying selection, reflecting its fundamental role in cellular metabolism (OpenTargets Search: -PHYKPL, liang2019currentadvanceson pages 19-20). Comparative studies and variant analyses, such as those incorporated in UniProt and Open Targets platforms, support the notion that missense variants affecting PHYKPL’s catalytic residues are deleterious, further emphasizing the necessity for conservation of its molecular features (OpenTargets Search: -PHYKPL). Furthermore, the localization to the mitochondrion—a compartment central to energy metabolism—is a feature that is conserved not only in mammals but also across other eukaryotic systems, highlighting the universal requirement for efficient control of amino acid catabolism in mitochondrial biology (rosso2008mitochondrialtargetingadaptation pages 8-8, key2023translationfidelityand pages 34-35). Such evolutionary persistence underlines the enzyme’s importance in fundamental metabolic processes and reinforces the rationale for its inclusion in rigorous GO annotation curation.
Key Experimental Evidence and Literature
A wealth of experimental evidence underpins the functional assignment of PHYKPL. Genetic association studies, including genome-wide association studies (GWAS) and analyses of missense variants, have provided high-confidence evidence linking alterations in PHYKPL to erythrocyte phenotypes and phosphohydroxylysinuria (OpenTargets Search: -PHYKPL). In addition, biochemical characterizations confirm the PLP dependency of the enzyme, as is typical for its class, and support the notion that it functions as a homotetramer (liang2019currentadvanceson pages 19-20, key2023translationfidelityand pages 34-35). While direct structural data such as high-resolution crystallography results may not yet be available specifically for PHYKPL, its classification based on enzyme commission number EC 4.2.3.134 and sequence comparisons with homologous PLP-dependent enzymes serve as robust indirect evidence for its molecular mechanism. Furthermore, transcriptomic and cis-eQTL studies in liver and adipose tissues provide regulatory evidence, indicating that natural genetic variation at the PHYKPL locus influences its expression and that such variation is associated with alterations in lipid metabolism and blood cell parameters (norheim2017geneticandhormonal pages 26-29). Taken together, these diverse lines of evidence—from genetic, biochemical, and bioinformatic investigations—strongly validate the functional, cellular, and clinical annotations of PHYKPL.
Conclusion
In summary, the Homo sapiens gene PHYKPL (UniProt: Q8IUZ5, AT2L2_HUMAN) encodes the mitochondrial enzyme 5‐phosphohydroxy‐L‐lysine phospho‐lyase, which plays a pivotal role in lysine catabolism by catalyzing a PLP-dependent breakdown reaction. The enzyme is localized in the mitochondrial matrix, where it assembles as a homotetramer to facilitate efficient substrate processing. It is primarily involved in key biological processes such as amino acid degradation, and genetic studies have robustly linked functional variation in PHYKPL to red blood cell traits and the rare disorder phosphohydroxylysinuria. Structural features such as conserved PLP-binding domains are critical for its catalytic mechanism, and even though high-resolution structural data may still be forthcoming, comparative analyses with related enzymes support its classification within this enzyme family. Expression patterns show regulated and tissue-specific transcript levels that correlate with metabolic phenotypes, and evolutionary conservation underscores its importance in maintaining cellular homeostasis across mammals. Overall, high-quality experimental evidence—including genetic association studies, missense variant analysis, and transcriptomic investigations—strongly supports the functional and biological role of PHYKPL, making it a significant target for further research and a valuable candidate for detailed Gene Ontology annotation (OpenTargets Search: -PHYKPL, liang2019currentadvanceson pages 19-20, rosso2008mitochondrialtargetingadaptation pages 8-8, key2023translationfidelityand pages 34-35, zhang2020pofut1isdispensable pages 7-9, norheim2017geneticandhormonal pages 26-29).

Key Gene Ontology terms that are applicable based on this synthesis include:
• Molecular Function: pyridoxal phosphate binding; 5-phosphohydroxy-L-lysine phospho-lyase activity; and enzyme activator activity (reflecting its PLP dependency).
• Cellular Component: mitochondrial matrix; and homotetrameric protein complex.
• Biological Process: lysine catabolic process; amino acid metabolic process; and regulation of erythrocyte development.

This report thus integrates diverse lines of evidence and literature data, providing a robust framework for the GO annotation curation of PHYKPL as a key enzyme in mitochondrial lysine metabolism with significant physiological and clinical relevance.

References

(OpenTargets Search: -PHYKPL): Open Targets Query (-PHYKPL, 4 results). Buniello, A. et al. (2025). Open Targets Platform: facilitating therapeutic hypotheses building in drug discovery. Nucleic Acids Research.
(liang2019currentadvanceson pages 19-20): Jing Liang, Qian Han, Yang Tan, Haizhen Ding, and Jianyong Li. Current advances on structure-function relationships of pyridoxal 5′-phosphate-dependent enzymes. Frontiers in Molecular Biosciences, Mar 2019. URL: https://doi.org/10.3389/fmolb.2019.00004, doi:10.3389/fmolb.2019.00004. This article has 130 citations and is from a poor quality or predatory journal.
(rosso2008mitochondrialtargetingadaptation pages 8-8): Lia Rosso, Ana C. Marques, Andreas S. Reichert, and Henrik Kaessmann. Mitochondrial targeting adaptation of the hominoid-specific glutamate dehydrogenase driven by positive darwinian selection. PLoS Genetics, 4:e1000150, Aug 2008. URL: https://doi.org/10.1371/journal.pgen.1000150, doi:10.1371/journal.pgen.1000150. This article has 55 citations and is from a domain leading peer-reviewed journal.
(key2023translationfidelityand pages 34-35): Jana Key, Suzana Gispert, Gabriele Koepf, Julia Steinhoff-Wagner, Marina Reichlmeir, and Georg Auburger. Translation fidelity and respiration deficits in clpp-deficient tissues: mechanistic insights from mitochondrial complexome profiling. International Journal of Molecular Sciences, 24:17503, Dec 2023. URL: https://doi.org/10.3390/ijms242417503, doi:10.3390/ijms242417503. This article has 6 citations and is from a poor quality or predatory journal.
(zhang2020pofut1isdispensable pages 7-9): S Zhang, Q Yang, Z Liu, and S Shi. Pofut1 is dispensable for structure, function and survival of mouse podocytes. Unknown journal, 2020.
(norheim2017geneticandhormonal pages 26-29): Frode Norheim, Simon T. Hui, Emre Kulahcioglu, Margarete Mehrabian, Rita M. Cantor, Calvin Pan, Brian W. Parks, and Aldons J. Lusis. Genetic and hormonal control of hepatic steatosis in female and male mice. Journal of Lipid Research, 58:178-187, Jan 2017. URL: https://doi.org/10.1194/jlr.m071522, doi:10.1194/jlr.m071522. This article has 80 citations and is from a peer-reviewed journal.

📄 View Raw YAML

id: Q8IUZ5
gene_symbol: PHYKPL
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: 'PHYKPL encodes 5-phosphohydroxy-L-lysine phospho-lyase, a mitochondrial enzyme that catalyzes

  the pyridoxal phosphate-dependent breakdown of 5-phosphohydroxy-L-lysine into ammonia, inorganic

  phosphate, and 2-aminoadipate semialdehyde. This enzyme functions as a key component in the

  catabolism of hydroxylysine residues derived from collagen degradation, connecting extracellular

  matrix turnover to lysine catabolism and energy metabolism. Despite being classified in the

  aminotransferase III family, PHYKPL functions as an ammoniophospholyase rather than a transaminase.

  Loss-of-function mutations cause phosphohydroxylysinuria, a benign metabolic condition characterized

  by elevated urinary 5-phosphohydroxylysine without apparent clinical symptoms.

  '
existing_annotations:
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  review:
    summary: |
      Annotated as mitochondrial based on UniProt subcellular location
      (ECO:0000305 curator inference). NOTE: PMID:22241472 (Veiga-da-Cunha
      2012) did NOT experimentally confirm mitochondrial localization — it
      reported a TargetP/PSORT II *prediction* of cytosolic localization
      and noted the contradiction with the curated mitochondrial assignment.
      Per PR #749 review feedback, the earlier summary's claim of
      "experimental confirmation" of mitochondrial localization was
      incorrect. The ACCEPT action remains defensible because the protein
      handles a metabolite (5-phosphohydroxy-L-lysine) catabolized via
      pathways with mitochondrial counterparts and UniProt's curator
      inference is reasonable, but the annotation is currently supported
      by prediction/curator inference rather than direct microscopy or
      proteomics evidence.
    action: ACCEPT
    supported_by:
    - reference_id: file:human/PHYKPL/PHYKPL-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Mitochondrion {ECO:0000305|PubMed:22241472}'
    - reference_id: file:human/PHYKPL/PHYKPL-deep-research-lite.md
      supporting_text: See deep research file for comprehensive analysis
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: 'Correctly annotated. The mitochondrial matrix localization is appropriate for this

      enzyme that participates in amino acid catabolism, and is consistent with the

      experimental characterization in PMID:22241472.

      '
    action: ACCEPT
    supported_by: []
- term:
    id: GO:0008483
    label: transaminase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: 'Incorrect annotation. PMID:22241472 explicitly states: "Unlike AGXT2, AGXT2L1 and

      AGXT2L2 did not act as transaminases." Despite belonging to the aminotransferase III

      family, PHYKPL functions as an ammoniophospholyase, not a transaminase. The UniProt

      entry also cautions: "Does not seem to possess aminotransferase activity."

      '
    action: REMOVE
    supported_by:
    - reference_id: PMID:22241472
      supporting_text: Unlike AGXT2, AGXT2L1 and AGXT2L2 did not act as transaminases
- term:
    id: GO:0016829
    label: lyase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000043
  review:
    summary: 'Too general. The enzyme is indeed a lyase (EC 4.2.3.134), but more specifically

      a carbon-oxygen lyase acting on phosphates. PMID:22241472: "The phosphorylation product

      made by this enzyme was metabolized by AGXT2L2, which converted it to ammonia,

      inorganic phosphate, and 2-aminoadipate semialdehyde."

      '
    action: MODIFY
    proposed_replacement_terms:
    - id: GO:0016838
      label: carbon-oxygen lyase activity, acting on phosphates
- term:
    id: GO:0030170
    label: pyridoxal phosphate binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: 'Correctly annotated. PMID:22241472: "AGXT2L1 and AGXT2L2 catalyzed the

      pyridoxal-phosphate-dependent breakdown of phosphoethanolamine and

      5-phosphohydroxy-L-lysine." The enzyme requires PLP as an essential cofactor.

      '
    action: ACCEPT
    supported_by:
    - reference_id: PMID:22241472
      supporting_text: putative pyridoxal-phosphate-dependent enzymes encoded by vertebrate genomes
    - reference_id: file:human/PHYKPL/PHYKPL-deep-research-falcon.md
      supporting_text: AGXT2L2 was purified as a **yellow PLP-binding protein**, consistent with PLP cofactor association
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:21044950
  review:
    summary: 'Generic protein binding annotation from a high-throughput screen (YFP complementation

      for telomere signaling regulators). The interaction partner is POT1 (telomere protein),

      which seems biologically unlikely given PHYKPL''s mitochondrial localization and metabolic

      function. This is likely a false positive or artifact. The generic "protein binding"

      term provides no functional insight.

      '
    action: REMOVE
    supported_by:
    - reference_id: PMID:21044950
      supporting_text: Epub 2010 Nov 2. Genome-wide YFP fluorescence complementation screen identifies new regulators for telomere signaling in human cells.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25416956
  review:
    summary: 'Generic protein binding from proteome-scale interactome mapping. Shows interactions

      with USO1 (vesicle transport) and VAC14 (PI signaling), which are not obviously

      related to PHYKPL''s metabolic function. These high-throughput interactions lack

      validation and the generic term provides no functional insight.

      '
    action: REMOVE
    supported_by:
    - reference_id: PMID:25416956
      supporting_text: A proteome-scale map of the human interactome network.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:29892012
  review:
    summary: 'Generic protein binding from an interactome perturbation study. Shows interactions

      with VAC14 and LNX2, neither obviously related to lysine catabolism. High-throughput

      data without validation, and the generic term adds no functional information.

      '
    action: REMOVE
    supported_by:
    - reference_id: PMID:29892012
      supporting_text: Jun 11. An interactome perturbation framework prioritizes damaging missense mutations for developmental disorders.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:31515488
  review:
    summary: 'Generic protein binding from a study on genetic variants disrupting protein interactions.

      Shows interaction with LNX2, not obviously related to PHYKPL function. Generic term

      provides no functional insight.

      '
    action: REMOVE
    supported_by:
    - reference_id: PMID:31515488
      supporting_text: Extensive disruption of protein interactions by genetic variants across the allele frequency spectrum in human populations.
- term:
    id: GO:0042802
    label: identical protein binding
  evidence_type: IPI
  original_reference_id: PMID:16189514
  review:
    summary: 'Self-interaction detected in high-throughput screen. This is biologically plausible

      as UniProt states: "Homotetramer" based on similarity to related enzymes. The

      tetramerization is likely functionally relevant for enzymatic activity, as is

      common for PLP-dependent enzymes.

      '
    action: ACCEPT
    supported_by:
    - reference_id: file:human/PHYKPL/PHYKPL-uniprot.txt
      supporting_text: 'Subunit: Homotetramer. {ECO:0000250|UniProtKB:P22256}'
    - reference_id: PMID:16189514
      supporting_text: Towards a proteome-scale map of the human protein-protein interaction network.
- term:
    id: GO:0042802
    label: identical protein binding
  evidence_type: IPI
  original_reference_id: PMID:25416956
  review:
    summary: 'Self-interaction confirmed in proteome-scale interactome map. Consistent with

      the known homotetramer structure of PHYKPL, supporting the functional oligomerization.

      '
    action: ACCEPT
    supported_by:
    - reference_id: PMID:25416956
      supporting_text: A proteome-scale map of the human interactome network.
- term:
    id: GO:0042802
    label: identical protein binding
  evidence_type: IPI
  original_reference_id: PMID:31515488
  review:
    summary: 'Self-interaction detected in genetic variant study. Consistent with homotetramer

      formation required for enzymatic function.

      '
    action: ACCEPT
    supported_by:
    - reference_id: PMID:31515488
      supporting_text: Extensive disruption of protein interactions by genetic variants across the allele frequency spectrum in human populations.
- term:
    id: GO:0042802
    label: identical protein binding
  evidence_type: IPI
  original_reference_id: PMID:32296183
  review:
    summary: 'Self-interaction confirmed in reference human binary interactome map. Further

      supports the homotetramer structure necessary for enzymatic function.

      '
    action: ACCEPT
    supported_by:
    - reference_id: PMID:32296183
      supporting_text: Apr 8. A reference map of the human binary protein interactome.
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5696408
  review:
    summary: 'Original Reactome pathway reference cannot be validated due to missing cached file. However, mitochondrial matrix localization is correct based on other evidence.

      '
    action: ACCEPT
- term:
    id: GO:0042219
    label: modified amino acid catabolic process
  evidence_type: IDA
  original_reference_id: PMID:22241472
  review:
    summary: 'PMID:22241472 demonstrates that PHYKPL catalyzes the breakdown of
      5-phosphohydroxylysine, a phosphorylated derivative of hydroxylysine from collagen.
      Term updated from obsolete GO:0046948 (hydroxylysine catabolic process) to
      GO:0042219 (modified amino acid catabolic process) since the original term was
      obsoleted due to no evidence of the process existing.'
    action: NEW
    supported_by:
    - reference_id: PMID:22241472
      supporting_text: The phosphorylation product made by this enzyme was metabolized by AGXT2L2, which converted it to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde
    - reference_id: file:human/PHYKPL/PHYKPL-deep-research-falcon.md
      supporting_text: This reaction places PHYKPL in a defined pathway for **hydroxylysine catabolism** downstream of a 5-hydroxylysine kinase (AGPHD1/HYKK).
- term:
    id: GO:0016838
    label: carbon-oxygen lyase activity, acting on phosphates
  evidence_type: IDA
  original_reference_id: PMID:22241472
  review:
    summary: 'NEW ANNOTATION: More specific than general "lyase activity". PHYKPL is a phospho-lyase

      that cleaves carbon-oxygen bonds by eliminating phosphate, as demonstrated in PMID:22241472.

      '
    action: NEW
    supported_by:
    - reference_id: PMID:22241472
      supporting_text: AGXT2L2, which converted it to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde
    - reference_id: file:human/PHYKPL/PHYKPL-deep-research-falcon.md
      supporting_text: 'Kinetic parameters reported for recombinant human AGXT2L2 acting on 5PHyl include: - **Km(5PHyl) = 16.9 ± 4.5 μM** - **Vmax = 256 ± 15 nmol·min⁻¹·mg⁻¹ protein**'
references:
- id: PMID:22241472
  title: Molecular identification of hydroxylysine kinase and of ammoniophospholyases acting on 5-phosphohydroxy-L-lysine and phosphoethanolamine
  findings:
  - statement: PHYKPL/AGXT2L2 catalyzes PLP-dependent breakdown of 5-phosphohydroxy-L-lysine to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde, functioning downstream of AGPHD1/HYKK in a two-step hydroxylysine catabolism route.
    supporting_text: The phosphorylation product made by this enzyme was metabolized by AGXT2L2, which converted it to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde.
    reference_section_type: RESULTS
  - statement: Despite homology to class-III aminotransferases, AGXT2L2 does not catalyze transamination but instead acts as an ammoniophospholyase.
    supporting_text: Unlike AGXT2, AGXT2L1 and AGXT2L2 did not act as transaminases
    reference_section_type: INTRODUCTION
- id: PMID:23242558
  title: Mutations in the AGXT2L2 gene cause phosphohydroxylysinuria
  findings:
  - statement: Compound heterozygous mutations (p.Gly240Arg, p.Glu437Val) in AGXT2L2/PHYKPL cause phosphohydroxylysinuria; mutant recombinant proteins are largely insoluble, indicating loss of phosphohydroxylysine phospholyase activity in vivo. The condition does not appear to be a neurometabolic disease based on patient phenotype diversity.
    supporting_text: We conclude that phosphohydroxylysinuria is due to mutations in the AGXT2L2 gene and the resulting lack of activity of phosphohydroxylysine phospholyase in vivo.
    reference_section_type: ABSTRACT
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms.
  findings: []
- id: GO_REF:0000043
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt.
  findings: []
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings: []
- id: PMID:16189514
  title: Towards a proteome-scale map of the human protein-protein interaction network.
  findings: []
- id: PMID:21044950
  title: Genome-wide YFP fluorescence complementation screen identifies new regulators for telomere signaling in human cells.
  findings: []
- id: PMID:25416956
  title: A proteome-scale map of the human interactome network.
  findings: []
- id: PMID:29892012
  title: An interactome perturbation framework prioritizes damaging missense mutations for developmental disorders.
  findings: []
- id: PMID:31515488
  title: Extensive disruption of protein interactions by genetic variants across the allele frequency spectrum in human populations.
  findings: []
- id: PMID:32296183
  title: A reference map of the human binary protein interactome.
  findings: []
- id: Reactome:R-HSA-5696408
  title: Reactome pathway (cached file not available)
  findings: []
- id: file:human/PHYKPL/PHYKPL-deep-research-lite.md
  title: Deep research on PHYKPL function
  findings: []
- id: file:human/PHYKPL/PHYKPL-deep-research-falcon.md
  title: Falcon deep research synthesis on PHYKPL/AGXT2L2 function, kinetics, and pathway placement
  findings:
  - statement: PHYKPL acts as a PLP-dependent ammoniophospholyase downstream of AGPHD1/HYKK in hydroxylysine catabolism, with strong primary-biochemistry evidence anchored in Veiga-da-Cunha 2012.
    supporting_text: PHYKPL (synonym **AGXT2L2**) encodes a **pyridoxal-5′-phosphate (PLP)-dependent ammoniophospholyase** whose primary, experimentally demonstrated function is to catalyze elimination of **5-phosphohydroxy-L-lysine (5PHyl)** to yield **2-aminoadipate semialdehyde (2-AASA)**, **inorganic phosphate (Pi)**, and **ammonia (NH3)**.
  - statement: Recombinant human PHYKPL/AGXT2L2 shows Km of 16.9 ± 4.5 μM and Vmax of 256 ± 15 nmol/min/mg for 5PHyl, with substrate inhibition above ~100 μM and inhibition by inorganic phosphate.
    supporting_text: 'Kinetic parameters reported for recombinant human AGXT2L2 acting on 5PHyl include: - **Km(5PHyl) = 16.9 ± 4.5 μM** - **Vmax = 256 ± 15 nmol·min⁻¹·mg⁻¹ protein**'
  - statement: Localization was originally reported as cytosolic based on TargetP/Psort II predictions in the 2012 paper, but UniProt subsequently curated PHYKPL as mitochondrial; falcon noted this is a prediction rather than direct microscopy/proteomics evidence.
    supporting_text: AGXT2L2 (PHYKPL) is described as a **cytosolic enzyme**, consistent with **TargetP** and **Psort II** predictions cited in the biochemical characterization paper.
core_functions:
- description: Catalyzes the pyridoxal phosphate-dependent cleavage of 5-phosphohydroxy-L-lysine derived from collagen degradation
  molecular_function:
    id: GO:0016838
    label: carbon-oxygen lyase activity, acting on phosphates
  locations:
  - id: GO:0005759
    label: mitochondrial matrix
  directly_involved_in:
  - id: GO:0042219
    label: modified amino acid catabolic process
  supported_by:
  - reference_id: PMID:22241472
    supporting_text: The phosphorylation product made by this enzyme was metabolized by AGXT2L2, which converted it to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde
proposed_new_terms:
- proposed_name: 5-phosphooxy-L-lysine phospho-lyase activity
  proposed_definition: Catalysis of the reaction (5R)-5-phosphooxy-L-lysine + H2O = (S)-2-amino-6-oxohexanoate + NH4+ + phosphate
  justification: Specific molecular function term needed for PHYKPL enzymatic activity (EC 4.2.3.134), currently only general lyase term available
- proposed_name: 5-phosphohydroxy-L-lysine catabolic process
  proposed_definition: The chemical reactions and pathways resulting in the breakdown of 5-phosphohydroxy-L-lysine, a phosphorylated derivative of hydroxylysine
  justification: Specific metabolic process for this branch of lysine catabolism involving collagen-derived hydroxylysine
suggested_questions:
- question: What regulates PHYKPL expression under different metabolic conditions or during collagen remodeling?
  experts:
  - Metabolic biochemists
  - ECM biologists
- question: Are there tissue-specific differences in PHYKPL expression or activity that correlate with collagen turnover rates?
  experts:
  - Tissue metabolism specialists
- question: Why does phosphohydroxylysinuria have no apparent clinical phenotype despite metabolite accumulation?
  experts:
  - Clinical geneticists
  - Metabolic disease specialists
- question: Does PHYKPL interact with other enzymes in the lysine catabolic pathway to form metabolic complexes?
  experts:
  - Mitochondrial biochemists
- question: What is the structural basis for the enzyme's substrate specificity and PLP-dependent mechanism?
  experts:
  - Structural biologists
  - Enzymologists
suggested_experiments:
- experiment_type: Structural biology
  description: Crystallize human PHYKPL with substrate analogs and PLP to determine 3D structure
  hypothesis: Structure will reveal unique active site features explaining substrate specificity for phosphohydroxylysine
- experiment_type: Metabolomics
  description: Compare metabolite profiles in PHYKPL knockout vs wild-type cells under high collagen turnover conditions
  hypothesis: Loss of PHYKPL will lead to accumulation of 5-phosphohydroxylysine and reveal alternative catabolic routes
- experiment_type: Proteomics
  description: Identify PHYKPL protein interaction partners in mitochondrial matrix using proximity labeling (BioID)
  hypothesis: PHYKPL forms complexes with other lysine catabolic enzymes for efficient substrate channeling
- experiment_type: Expression profiling
  description: Analyze PHYKPL expression across tissues and developmental stages using single-cell RNA-seq
  hypothesis: PHYKPL expression correlates with tissue collagen content and turnover rates
- experiment_type: Metabolic flux analysis
  description: Use 13C-labeled hydroxylysine to trace flux through PHYKPL pathway in various cell types
  hypothesis: Flux through PHYKPL varies with cell type and metabolic state
status: COMPLETE

PHYKPL

Gene Description

Proposed New Ontology Terms

5-phosphooxy-L-lysine phospho-lyase activity

5-phosphohydroxy-L-lysine catabolic process

Existing Annotations Review

Core Functions

Catalyzes the pyridoxal phosphate-dependent cleavage of 5-phosphohydroxy-L-lysine derived from collagen degradation

References

Suggested Questions for Experts

Suggested Experiments

Deep Research

Falcon

Research report: Human PHYKPL (AGXT2L2; UniProt Q8IUZ5)

Executive summary

1) Key concepts and definitions (current understanding)

1.1 Gene/protein identity and nomenclature

1.2 Enzyme class: PLP-dependent “ammoniophospholyase”

1.3 Reaction and pathway placement: hydroxylysine catabolism

2) Functional biochemistry: substrate specificity and kinetics

2.1 Experimental evidence for substrate specificity

2.2 Quantitative kinetics and inhibition

3) Cellular/subcellular localization and expression

3.1 Subcellular localization

3.2 Tissue expression and physiological context

4) Disease associations, applications, and real-world implementations

4.1 Mendelian/metabolic disease: phosphohydroxylysinuria

4.2 Complex disease genetics (database-level evidence)

4.3 Clinical trials and therapeutics

5) Recent developments and latest research (prioritizing 2023–2024)

5.1 2024: broader PLP-enzyme/mitochondrial metabolism discussion

5.2 2023–2024 gaps in the retrievable corpus

6) Expert interpretation and analysis (authoritative sources)

7) Key statistics and data points from studies

Evidence map (summary table)

URLs and publication dates (key sources)

Limitations of this review

Artifacts

Citations

Lite

PHYKPL (5-Phosphohydroxylysine Phospholyase): Enzymatic Function

Role in Cellular and Biological Processes

Gaps in Current Knowledge

Questions for an Expert

Experimental Approaches to Elucidate Function

📚 Additional Documentation

Falcon Research

Falcon Research Report: PHYKPL (human)

📄 View Raw YAML